The invention relates to a method of detecting a watermark in an information signal that has possibly been watermarked by modifying values of said information signal in accordance with (temporally or spatially) corresponding values of a watermark pattern. The invention also relates to an arrangement for detecting a watermark.
A prior art method as defined in the opening paragraph is disclosed in International Patent Application WO-A-98/03014. The watermark is detected by computing the correlation of the suspect information signal with an applied watermark pattern, and comparing the correlation with a predetermined threshold. If the correlation is larger than the threshold, the watermark is said to be present, otherwise it is said to be absent.
It is an object of the invention to provide a suitable criterion for setting the threshold.
To this end, the invention provides a method of detecting a given watermark in an information signal, comprising the steps of: computing the correlation of said watermark and said information signal for a plurality of positions of said watermark with respect to said information signal; and detecting whether at least one of the respective correlation values exceeds a given threshold; characterized in that the method comprises the step of determining the standard deviation of the respective correlation values, wherein said given threshold is a given multiple of said standard deviation.
The invention exploits the insight that watermark detectors need to compute the correlation value for a plurality of (temporal or spatial) positions of the watermark with respect to the information signal (for example, an image) in practice, because the position of the watermark with respect to image is not absolutely known and/or information is embedded in shifts of one or more watermark patterns. The detection thus yields a series of correlation values, and it is the occurrence of relative peaks in such a series which is of interest rather than absolute correlation values.